1. Field of the Invention
This invention relates to nonlinear optical devices and process for changing the frequency of electromagnetic radiation and, more particularly, to such devices in which the crystal therein, having nonlinear optical properties, consists essentially of a lanthanum, europium or gadolinium hydroxide fluoride carbonate.
2. Description of the Prior Art
Electrooptic devices utilizing the non-zero components of the second order polarizability tensor to achieve second harmonic generation (SHG), parametric amplification, the addition and subtraction of frequencies, tunable frequencies, modulation and the like of coherent electromagnetic radiation have been described by Albert A. Ballman, Gary D. Boyd and Robert C. Miller in U.S. Pat. No. 3,262,058, by J. A. Giordmaine and Robert C. Miller in U.S. Pat. No. 3,328,723, by Satoshi Nanamatsu and Masakazu Kimura in U.S. Pat. No. 3,747,022 and by John D. Bierlein and Thurman E. Gier in U.S. Pat. No. 3,949,323. A comprehensive review of nonlinear devices is given in Quantum Electronics Vols. 1A and 1B, edited by H. Rabin and C. L. Tang, Academic Press, New York, San Francisco and London (1975).
Briefly, electromagnetic waves propagating in a crystal that has nonlinear optical properties induce polarization waves having frequencies which are the sum and the difference of the frequencies of the exciting waves. These polarization waves can radiate electromagnetic waves having the frequencies of the polarization waves. The energy transferred to a radiated electromagnetic wave from a polarization wave depends on the magnitude of the component of the second order polarizability tensor involved, since this tensor element determines the amplitude of the polarization wave, and also on the distance over which the polarization wave and the radiated electromagnetic wave remain sufficiently in phase, called the coherence length. The coherence length is given by .pi./(.DELTA..kappa.) wherein .DELTA..kappa. is the difference between the wave vector of the radiated electromagnetic wave and the wave vector of the polarization wave. Phase matching occurs when the waves are completely in phase, that is when .DELTA..kappa.=0. The condition .DELTA..kappa.=0 can also be expressed as n.sub.3 .omega..sub.3 =n.sub.1 .omega..sub.1 .+-.n.sub.2 .omega..sub.2 wherein .omega..sub.3 =.omega..sub.1 .+-..omega..sub.2 and where .omega..sub.1 and .omega..sub.2 are the frequencies of the exciting light and .omega..sub.3 is that of the radiated optical wave and n.sub.1, n.sub.2 and n.sub.3 are the corresponding refractive indices. The plus signs are appropriate when the sum frequency is the one of interest; the minus signs are appropriate when the difference frequency is the one of interest.
A particular case which serves as an example of nonlinear effects is second harmonic generation (SHG) where only one frequency .omega. of the incident beam serves as the exciting wave and .omega..sub.1 =.omega..sub.2 =.omega. and .omega..sub.3 =2.omega.. As a result of the interaction of the incident beam of radiation with the crystal having nonlinear optical properties, the radiated wave emerging from the crystal contains at least one frequency different from any frequency contained in the incident beam.
The above phase matching conditions can be met with birefringent crystals provided the refractive index difference between the ordinary and the extraordinary rays is sufficiently large to offset the change of refractive index with frequency, i.e., optical dispersion.
Generally phase matching is of two types:
Type I wherein the two incident waves have the same polarizations and PA0 Type II wherein the two incident waves have orthogonal polarizations. PA0 Phase matching can be achieved by "tuning" the crystal in various ways:
1. By rotation of the crystal to vary the refractive indices. PA1 2. By varying the temperature. PA1 4. By compositional variation.
3. By application of an electric field.
The possibility of achieving one or more types of phase matching and the appropriate orientation of the crystal to the incident wave depends on the existence of non-zero elements in the second order polarizability tensor. Depending on the point group symmetry of the crystal some elements will be identically zero, while equalities are imposed on other elements. The magnitude of the effects will depend on the magnitude of the non-zero elements.
Of particular interest are crystals which are capable of operating in the ultraviolet range of the electromagnetic spectrum. Since powerful and efficient lasers emitting at 1.06 .mu.m are readily available, and materials capable of doubling the frequency of this radiation now approach efficiencies of 50%, a material capable of redoubling the frequency doubled output of such a laser would be particularly attractive.
Many materials which are recognized as being efficient nonlinear optical materials are excluded from use in this frequency range because they either absorb in the ultraviolet or the birefringence is not large enough in this special range to permit phase matching. Of the materials which have been shown to operate in the ultraviolet, none is without serious limitations. The best of these are members of the potassium dihydrogen phosphate (KDP) family which are temperature sensitive, water soluble, and difficult to work with and KB.sub.5 O.sub.8.4H.sub.2 O which is extremely water sensitive and has small nonlinear optical coefficients. Other materials are even more deficient.
Accordingly, there is a need in the art to provide crystals which are phase matchable in the ultraviolet and which overcome the serious limitations of the known materials which operate in this frequency range.
Rare earth hydroxide carbonates, rare earth fluoride carbonates and rare earth hydroxide fluoride carbonates, their hydrothermal preparation and determination of their crystal structures have been reported in the literature [see, for example, Ya. D. Fridman et al., Russ. J. Inorg. Chem. 14 (10), 1440 (1969); A. N. Christensen, Acta Chem. Scand. 27, 2973 (1973); J. M. Haschke, J. Solid State Chem., 12, 115 (1975); M. P. Caro et al., Comptes rendus 272, Series C, 57 (1971); M. R. Aumont et al., Comptes rendus 272, Series C, 314 (1971)]. S. K. Kurtz et al., J. Appl. Phys. 39, 3798 (1968), disclose CeFCO.sub.3 as a material which has nonlinear coefficients equal to or less than that of quartz and which has no phase-matchable directions for second harmonic generation for an input wave-length of 1.06 .mu.m. The authors conclude that this material is not potentially useful in a nonlinear optical device. The art appears to be devoid of any other disclosure of the nonlinear optical properties of LnF.sub.x (OH).sub.1-x CO.sub.3 and the use thereof in a nonlinear optical device.